1. Field of the Invention
This invention relates to a structure of a sheet feeding device for an image forming apparatus such as a laser printer, a copying machine or a facsimile apparatus. More particularly, this invention relates to a structure of a sheet feeding device wherein recording medium layered in a sheet feeding station, such as a sheet feed cassette are separated and fed one by one from the top position thereof.
2. Description of the Related Art
Various sheet feeding devices for an image forming apparatus such as a laser printer for feeding cut sheets of a predetermined size are known. An example of a conventional sheet feeding device is the frictional separation type device disclosed in Japanese Unexamined Patent Publication No. 63-37044.
In particular, in the frictional separation type device, a large number of sheets are received on a receiving plate normally biased upwardly in a sheet feed cassette, and a sheet feed shaft, which is driven to rotate intermittently, is disposed adjacent to an end portion above an uppermost one of the sheets. A sheet feed roller having a substantially D-shaped cross section and having a circumferential portion formed of a member having a high coefficient of friction is secured to an intermediate portion of the sheet feed shaft. A pair of cylindrical sheet feed collars having a diameter slightly smaller than that of the circumferential portion of the sheet feed roller are fitted for rotation at the opposite end portions of the sheet feed roller. When the sheet feed shaft is driven to rotate by a predetermined angle by a motor, the circumferential portion of the sheet feed roller is contacted under pressure with the surface of the sheet at the uppermost position so that the uppermost sheet is pushed out forwardly (downstream in the feeding direction) by the sheet feed roller due to the friction of the circumferential portion of the sheet feed roller.
When the uppermost sheet is pushed out, also other sheets underlying the uppermost sheet are dragged so that a plurality of sheets are sometimes pushed out at a time. Such phenomenon is called "dual feed." In order to prevent such dual feed, a separation pad holder made of a synthetic resin is conventionally disposed at a location at the height of the uppermost sheet immediately downstream of the sheet in the sheet feeding direction on the side on which it is opposed to the circumferential face of the sheet feed roller. The separation pad holder is provided, at an upper face thereof (the side opposed to the circumferential face of the sheet feed roller), with a separation pad member having a coefficient of friction a little lower than that of the material of the sheet feed roller. The separation pad roller is normally biased by a spring so that it approaches the circumferential face of the sheet feed roller.
When the circumferential portion of the sheet feed roller being driven to rotate is opposed to the surface of the separation pad, the sheet feed roller pushes down the separation pad roller against the biasing force of the spring by the sheets. In this condition, the uppermost sheet is pushed out downstream in the feeding direction by the frictional force of the circumferential portion of the sheet feed roller. Meanwhile, since resistance acts on the other sheets underlying the uppermost sheet to prevent them from being pushed out by the friction of the separation pad, only the uppermost (single) sheet is pushed out and fed to a pair of transport sheet rollers located downstream in the feeding direction with respect to the separation pad holder.
It is to be noted that when the sheet feed roller is rotated by a predetermined angle until a flat portion thereof having a smaller diameter is opposed to the separation pad holder, the outer circumferential faces of the sheet feed collars greater in diameter than the flat portion of the sheet feed roller and freely rotatable with respect to the sheet feed roller contact with the surface of the uppermost sheet sent out while the rear face of the sheet being sent out contacts with the separation pad.
However, in the above structure, the separation pad is provided both in the upstream and the downstream of the sheet feeding direction from a portion where the sheet feed roller and the separation pad contact each other. Therefore, while the uppermost sheet separated from the other underlying sheets by the sheet feed roller and the separation pad is being sent out by the transport roller pair, the uppermost sheet being sent out remains in a condition where it is contacted under pressure by and between the sheet feed collars and the separation pad. Consequently, the rear face of the uppermost sheet being sent out rubs on the surface of the separation pad and some play is present at a location where the separation pad is mounted for rocking motion so as to press against the circumferential face of the sheet feed roller. There is a problem in that when a sheet is fed, the sheet and/or the separation pad vibrate and generate noise, such as chattering.